Currently, no uniform system has been adopted to evaluate neutralizing antibody response to vaccines. Indicator T-cells (H938) and monocytes (U38) containing stably integrated copies of the reported gene HIV-LTR-CAT represent a rapid, quantitative basis for evaluating antibody response to vaccines. CAT signals produced in indicator T-cells exposed to H9-HIV-lllB cells for 2 days were quantitatively reduced when patient serum containing neutralizing antibodies were also present. Neutralizing titer of patient serum can be independently corroborated by rapid, sensitive assays for reverse transcriptase and p24 antigen in indicator cells. Several strains of HIV are being cultured with T-cells and both cells and cell-free virus are being cyopreserved and characterized by EM and reverse transcriptase activity for subsequent testing with human serum in the indicator cell assay. The site of antibody neutralization will be located by the ability of peptides to compete with antibodies in the cell assay. An array of peptides spanning the entire HIV gp120 region are available for mapping neutralizing epitopes. Ira Berkower of DBB, CBER, has extensively characterized neutralizing antibodies in patient serum using an indirect, 7 day, plaque assay he developed. Comparison of the indicator cell assay and the plaque assay using comparable conditions are scheduled. MicoGeneSys, Inc., has agreed to provide characterized serum samples that include data obtained in syncytia- forming and plaque assays for use in developing a neutralization test system. Results of these experiments will provide a basis for evaluating the methods available for accessing the protective value of vaccines.